In recent years, attention has been given to hybrid vehicles and electric vehicles against the background of environmental issues. The hybrid vehicle uses, as its drive power source, an electric motor driven by a DC power supply via an inverter in addition to a conventional engine. Thus, it obtains the drive power by driving the engine, and also obtains the drive power by such a manner that the inverter converts a DC voltage supplied from the DC power supply into an AC voltage, and the converted AV voltage drives the electric motor. The electric vehicle uses, as its drive power source, an electric motor driven by a DC power supply via an inverter.
An Intelligent Power Module (IPM) mounted on the hybrid vehicle or the electric vehicle is configured to perform fast switching in a semiconductor switching element (power semiconductor element) such as an IGBT (Insulated Gate Bipolar Transistor) and thereby converting the DC power supplied from the DC power supply into the AC power for driving the motor.
For example, Japanese Patent Laying-Open No. 2005-192296 has disclosed an inverter device having a plurality of inverter units each using a power device that is formed of a module capable of orthogonal three-phase power conversion. The plurality of inverter units have negative poles commonly connected to an integrally planar conductor (bus bar), and also have positive poles commonly connected to another integrally planar bus bar, respectively. The bus bar connected to the negative poles overlaps the bus bar connected to the positive poles with an insulator therebetween.
However, according to the above Japanese Patent Laying-Open No. 2005-192296, the plurality of inverter units share the bus bar, and this structure may suffer from a problem that a surge voltage generated in each inverter unit during a switching operation interferes with that of another inverter unit via the bus bar.
Thus, in the bus bar serving as a medium of electric power transmission and reception that are performed between the power supply and the plurality of inverter units, a surge voltage generated in each inverter unit is superimposed on the DC voltage to be originally transmitted or received. Therefore, when the DC voltage on which the surge voltage supplied from one of the inverter units is superimposed enters the other inverter unit via the bus bar, the level of the surge voltage may be amplified in the other inverter unit because the entered surge voltage may interference with the internally generated surge voltage. However, the above Japanese Patent Laying-Open No. 2005-192296 has not disclosed measures against such interference of the surge voltages.
For suppressing such amplification of the surge voltages, it is preferable to lower a switching speed corresponding to an open/close speed of the switching element in each inverter unit and thereby to reduce the surge voltage itself. However, the lowering of the switching speed increases the electric power loss that may occur in the switching operation.
Accordingly, the invention has been made for overcoming the above problems, and an object of the invention is to provide a power conversion device that can suppress the interference of surge voltages between the plurality of power conversion units.